1. Technical Field
The present disclosure generally relates to motion sensing elements and more particularly to geophone sensing elements.
2. Background Information
Geophones are used to sense motion in the earth. Geophones normally include a spring-mass sensing element to sense motion by suspending an inertial reference mass structure from a rigid, fixed supporting structure. Typically, the fixed supporting structure comprises an outer casing and a magnet, which is fixed inside the outer casing. This rigid, fixed supporting structure is typically fastened to the medium of which movement is to be measured using a housing structure including, for example, a spike. Typically, the sensing element reference mass is a coil assembly comprising a coil wound on a coilform and suspended by springs in a magnetic field, one spring being attached at each end of the coilform. The springs position the coil within the magnetic field so that the coil is centered laterally and along its axis within the magnetic field. The springs also form a suspension system having a predetermined resonant frequency. In general, the suspension system has a much lower resonant frequency in the direction along the main axis of the geophone sensing element than in the directions perpendicular to the main axis. The lower limit of the geophone sensing element frequency band is related to the resonant frequency along the main axis, and the upper limit of the geophone sensing element frequency band is related to the resonant frequency perpendicular to the main axis. Geophones are most useful when used within these upper and lower limit frequencies. Lowering the spring stiffness along the main axis reduces the resonant frequency in the direction along the main axis thereby widening the geophone useful frequency band. Many articles and skilled artisans use the term geophone synonymously with the coil-mass sensing element. The present disclosure is directed to the sensing element structure, although the term geophone may be used from time to time. The sensing element, however, may be used in movement sensing, vibration sensing and acceleration sensing in non-geophysical prospecting applications. Therefore, the term geophone is used merely for illustrative purposes and does not limit the scope of the present disclosure to geophysical applications.
In seismic operations, seismic waves are imparted into the earth's crust, and portions of those seismic waves are reflected or refracted from the boundaries of subsurface layers. Geophones are acoustically coupled to the earth, and when the reflected or refracted waves encounter a geophone, the coil assembly of the geophone sensing element, which coil assembly is suspended between the two springs, tends to stand still while the geophone housing and its connected magnetic circuit moves with the earth's surface. The movement of the coil assembly through a magnetic field causes a voltage to be generated at the output of the geophone. The output of the geophone or an array of geophones is recorded in a form which permits analysis. Skilled interpreters can discern from the analysis the shape of subsurface formations, and the likelihood of finding an accumulation of minerals, such as oil and gas.
In present day geophone sensing elements, spider springs are used extensively. Such springs are usually made from discs of spring material and have an inner ring and an outer ring which are connected by a plurality of legs. The legs are formed by etching or stamping the spring material in accordance with a predetermined pattern. Generally three such legs are used, and the three-legged arrangement is generally considered the most advantageous.
The legs of the springs generally have a rectangular cross-section, and are curved along their lengths between the junctures with the inner and outer rings of the spring. After etching, the spring may be “preformed” according to known techniques for geophones intended for use in a vertical orientation. When preforming is complete, the inner ring is offset or displaced relative to the outer ring, such that when a mass is suspended between two such springs, the inner ring, legs, and outer ring of each spring lie in the same plane, and the coil is centered in the magnetic field.
Sometimes a coil may be displaced such that it is not centered within the magnetic field. This displacement generally reduces the effectiveness and quality of the geophone. Such displacement may result from a change in the component of gravity along the main axis when the geophone is positioned in an orientation for which it was not designed. Such displacement may also result from movement of the body to which the geophone is connected. The effect of displacement is exacerbated when the spring stiffness along the main axis is lowered. Consequently, performing and spring stiffness impose practical limits on the width of a geophone useful frequency band.
Geophones have been proposed wherein a displacement sensor is used to determine the relative position of the inertial mass with respect to the support structure. Knowing the displacement of the mass is useful in determining gravity effect on the geophone, to determine whether the geophone is planted properly, and the information can be used in a circuit for providing force-balance feedback to the geophone. Displacement sensors are typically capacitive sensors, where one capacitor electrode is coupled to the inertial mass and a second capacitor electrode is coupled to and stationary with respect to the support structure. As the mass is displaced from an initial position, the distance between the capacitor electrode changes thereby changing the capacitance. The change in capacitance as measured and the measured change is used to determine the displacement of the inertial mass.
Attempts to provide inertial mass displacement sensing have heretofore proven difficult to implement in the manufacturing process for geophones due to the small size of today's geophone sensing element and the effect on sensitivity.